The present invention relates to a method for fabricating a semiconductor device. More particularly, it relates to a method for fabricating a semiconductor device including a step of subjecting a polymer film, which has been deposited on an insulating film through plasma etching using an etching gas including carbon and fluorine, to ashing using an oxygen gas or a gas including oxygen as a principal constituent.
In accordance with recently improved refinement of semiconductor integrated circuit devices, it has become necessary to form a contact hole with a smaller diameter. On the contrary, since the depth of a contact hole has not been largely changed, a technique to form a contact hole with a high aspect ratio (the depth of the contact hole/the diameter of the contact hole) has become necessary.
Also, since a resist film used for forming a hole pattern has been reduced in its thickness, it is significant how the value of (the depth of the contact hole)/(the thickness of the resist film to be etched) is increased, namely, how the value of resist selectivity (=(the etching rate of an insulating film used for forming the contact hole)/(the etching rate of the resist film) is increased.
For example, when the resist selectivity is not sufficiently high, most of the resist film is etched before forming the contact hole, and hence, the contact hole cannot be formed in a good shape. Specifically, the contact hole may have a trumpet-shaped upper opening or adjacent contact holes may be connected to each other because the resist film is removed.
As one method for attaining sufficiently high resist selectivity for forming a contact hole in a good shape, a rigid deposited film is formed by using, as an etching gas, a PFC (perfluorocarbon) gas with a high C/F ratio, such as a C2F6 gas (with a C/F ratio of 2/6), a C4F8 gas (with a C/F ratio of 4/8) or a C5F8 gas (with a C/F ratio of 5/8), or by employing carbon-rich etching conditions. Thus, high resist selectivity can be attained.
Recently, however, oxide film etching process with higher resist selectivity is employed, and hence, there is a problem that a sufficient etching rate of a polymer film formed on the resist film cannot be attained by conventional ashing even if the power is increased.
Further, ashing using an oxygen gas including a fluorine gas for attaining the etching rate invites surface roughness of the wafer or shaving of the underlying substrate.
Now, a conventional method for forming a contact hole will be described with reference to FIGS. 7A through 7C and 8A through 8C.
First, as shown in FIG. 7A, a resist pattern 12 having a contact hole opening is formed on a silicon oxide film 11 formed on an underlying layer 10 made from an etching stopper film of a silicon nitride film or the like, a plug of polysilicon, tungsten or the like, or a lower interconnect.
Next, as shown in FIG. 7B, an etching gas 13 including a fluorocarbon gas as a principal constituent is introduced into an etching chamber (not shown), so as to etch the silicon oxide film 11 by using the resist pattern 12 as a mask. In this manner, a contact hole 14 is formed in the silicon oxide film 11. Thus, a reaction product gas 15 of SiF4, CO2, H2O and others is generated and vaporized. At this point, a rigid polymer film 16 of (CxHyFz)n including, as a principal constituent, carbon or fluorine supplied from the plasma of the etching gas 13 is deposited on the top surface of the resist pattern 12, the bottom and the wall of the contact hole 14 and the inside wall of the etching chamber.
Then, as shown in FIG. 7C, an ashing gas 17 of an oxygen gas including a fluorocarbon gas is introduced into an ashing chamber (not shown), so as to ash the polymer film 16. Thus, oxygen activated by plasma generation power is bonded to carbon, that is, one principal constituent of the polymer film 16 so as to generate carbon dioxide, and fluorine is also vaporized. These gases are removed as a reaction product gas 18.
At this point, a residual polymer 19 is formed on the silicon oxide film 11 as shown in FIG. 8A. A large amount of activated oxygen with high energy is generated by the plasma generation power and the thus generated activated oxygen with high energy reaches the surface of the silicon oxide film 11. Therefore, fluorine included in the residual polymer 19 is concentrated and pushed into a surface portion of the silicon oxide film 11 by the activated oxygen reaching the surface of the silicon oxide film 11. As a result, a first fluorine implant layer 21 is formed in the surface portion of the silicon oxide film 11. Also, gas-phase fluorine included in the reaction product gas 18 is activated by the plasma generation power and reaches again the surface of the silicon oxide film 11, and then is implanted into a surface portion of the silicon oxide film 11. Therefore, a second fluorine implant layer 22 is formed in the surface portion of the silicon oxide film 11.
Furthermore, at this point, fluorine included in the polymer film 16 adhered onto the resist pattern 12 or included in the polymer film adhered onto the inside wall of the chamber and fluorine included in fluorocarbon added to the ashing gas also enters to reach the bottom of the contact hole 14. Therefore, a portion of the underlying layer 10 exposed within the contact hole 14 is etched to form a recess 23.
Next, as shown in FIG. 8B, the surface of the silicon oxide film 11 and the bottom of the contact hole 14 are wet cleaned with a cleaning agent 24, so as to remove the residual polymer 19.
Thus, the residual polymer 19 present on the surface of the silicon oxide film 11 and on the bottom of the contact bole 14 is completely removed in the wet cleaning. However, there is a difference in the etching rate in the wet cleaning between the surface portions of the silicon oxide film 11 where the first fluorine implant layer 21 and the second fluorine implant layer 22 are formed and a surface portion thereof where none of these implant layers are formed. Accordingly, irregularities are caused in the surface portions of the silicon oxide film 11, which results in surface roughness 25 as shown in FIG. 8C.
Furthermore, in removing the residual polymer 19 present on the surface of the silicon oxide film 11 and on the bottom of the contact hole 14 by the ashing, if large plasma generation power is applied for the ashing in order to attain a sufficient ashing rate or to definitely remove the residual polymer 19, fluorine included in the residual polymer 19 or fluorine included in the polymer film deposited on the inside wall of the chamber strikes the surface of the silicon oxide film 11. As a result, the surface roughness 25 caused in the wet cleaning is further increased.
Moreover, when a large amount of fluorine enters to reach the bottom of the contact hole 14 during the ashing as described above so as to form the recess 23 in the underlying layer 10 exposed within the contact hole 14 (as shown in FIG. 8A), there arises a problem of increase of contact resistance if the underlying layer 10 is an impurity diffusion layer. Alternatively, if the underlying layer 10 is an etching stopper film, a metal interconnect formed below the etching stopper film is exposed, and hence, the metal interconnect is oxidized by oxygen plasma or absorbs moisture. As a result, there arises a problem of degradation in the device characteristic.
In addition, the fluorine included in the polymer film 16 (shown in FIG. 7C) deposited on the surface of the silicon oxide film 11 and the fluorine generated from the fluorocarbon gas added to the ashing gas is activated by the plasma during the ashing, so as to damage parts of the chamber. As a result, there arises a problem of a short life of the parts.
Furthermore, the fluorine having struck the surface of the silicon oxide film 11 during the ashing may not be completely removed but remain through the cleaning. In this case, when a resist film of a chemically amplified resist material is formed on the silicon oxide film 11 having the contact hole 14 and the resist film is subjected to pattern exposure, the fluorine included in the first and second fluorine implant layers 21 and 22 deactivates an acid generated in an exposed portion of the resist film. As a result, there arises a problem that a resist pattern cannot be formed in a good shape.
In consideration of the aforementioned conventional problems, an object of the invention is, in removing a polymer film, which has been deposited on a resist pattern during plasma etching of an insulating film using an etching gas including carbon and fluorine, by ashing before wet cleaning the insulating film, preventing surface roughness from being caused in the insulating film.
In order to achieve the object, the first method for fabricating a semiconductor device of this invention comprises the steps of forming a resist pattern on an insulating film deposited on a semiconductor substrate and subjecting the insulating film to plasma etching using an etching gas including carbon and fluorine with the resist pattern used as a mask; performing a first stage of ashing on a polymer film having been deposited on the resist pattern during the plasma etching with a relatively low chamber pressure and relatively low plasma generation power by using an oxygen gas or a gas including oxygen as a principal constituent; and performing a second stage of the ashing on a residual polymer present on the insulating film in completing the first stage of the ashing with relatively high chamber pressure and relatively high plasma generation power by using an oxygen gas or a gas including oxygen as a principal constituent.
In the first method for fabricating a semiconductor device, since the polymer film is subjected to the first stage of the ashing with a relatively low chamber pressure and relatively low plasma generation power, fluorine included in a reaction product gas generating at the first stage of the ashing is minimally activated and activated oxygen has low energy. Therefore, fluorine included in the polymer film is minimally pushed into a surface portion of the insulating film by the activated oxygen or the fluorine included in the reaction product gas is minimally implanted into a surface portion of the insulating film. Accordingly, surface roughness can be prevented from being caused on the insulating film through wet cleaning subsequently performed.
Furthermore, since the residual polymer is subjected to the second stage of the ashing with a relatively high chamber pressure and relatively high plasma generation power, a large amount of activated oxygen with high energy is generated. As a result, the residual polymer is efficiently removed.
Even though a large amount of activated oxygen with high energy is generated at the second stage of the ashing, since the amount of fluorine included in the residual polymer is small, the implantation of fluorine into a surface portion of the insulating film can be suppressed.
The first method for fabricating a semiconductor device preferably further comprises, after the step of performing a second stage of the ashing, a step of performing a third stage of the ashing with a relatively low chamber pressure and relatively high plasma generation power under application of substrate bias power by using an oxygen gas or a gas including oxygen as a principal constituent.
Thus, the activated oxygen with high energy is widely distributed and pulled toward a recess, such as a bottom of a contact hole, by the substrate bias power. Therefore, even when the recess has a high aspect ratio, the polymer film remaining on the bottom of the recess can be removed.
In the first method for fabricating a semiconductor device, the third stage of the ashing is preferably performed with the chamber pressure set to 2.67 through 6.67 Pa, the plasma generation power set to 1000 through 3000 W and the substrate bias power set to 50 through 300 W.
Thus, even when the recess has a high aspect ratio, the polymer remaining on the bottom of the recess can be definitely removed.
In the first method for fabricating a semiconductor device, the first stage of the ashing is preferably performed with the chamber pressure set to 2.67 through 6.67 Pa and the plasma generation power set to 500 through 1000 W.
Thus, fluorine included in the reaction product gas generated at the first stage of the ashing can be definitely made to be minimally activated as well as the energy of the activated oxygen can be definitely made low. As a result, the surface roughness derived from fluorine can be definitely prevented.
In the first method for fabricating a semiconductor device, the second stage of the ashing is preferably performed with the chamber pressure set to 13.3 through 66.7 Pa and the plasma generation power set to 1000 through 3000 W.
Thus, a large amount of activated oxygen with high energy can be generated, and hence, the residual polymer can be more efficiently removed.
The first method for fabricating a semiconductor device is particularly effective when the plasma etching, the first stage of the ashing and the second stage of the ashing are performed within the same chamber.
In a conventional technique, if the plasma etching and the ashing are performed within the same chamber, fluorine included in a polymer film having been deposited on the inside wall of the chamber during the plasma etching is activated through the ashing so as to cause a variety of harmful influences. In contrast, according to the invention, the fluorine included in the polymer film deposited on the inside wall of the chamber is minimally activated, and hence, the harmful influences can be avoided.
The second method for fabricating a semiconductor device of this invention comprises the steps of depositing an insulating film on an underlying layer formed on a semiconductor substrate and depositing a peeling layer on the insulating film; forming a resist pattern on the peeling layer and subjecting the peeling layer and the insulating film to plasma etching using an etching gas including carbon and fluorine with the resist pattern used as a mask; performing ashing on a polymer film having been deposited on the resist pattern during the plasma etching by using an oxygen gas or a gas including oxygen as a principal constituent; and removing the peeling layer having, in a surface portion thereof, a fluorine implant layer having been formed during the ashing.
In the second method for fabricating a semiconductor device, since the peeling layer is formed on the insulating film, fluorine included in the polymer film is implanted into the peeling layer but not implanted into the insulating film below. Also, the fluorine implant layer formed in the surface portion of the peeling layer is completely removed together with the peeling layer. Accordingly, surface roughness derived from fluorine is never caused in the insulating film through wet cleaning subsequently performed.
Furthermore, since the fluorine implant layer is completely removed, in the case where a resist film of a chemically amplified resist material is formed on the insulating film after removing the peeling layer and the resist film is subjected to pattern exposure, an acid generated in an exposed portion of the resist film can be avoided from being deactivated through a reaction between an acid (H+) included in the chemically amplified resist material and fluorine.
In the second method for fabricating a semiconductor device, it is preferred that the insulating film is made from a silicon oxide film doped with no impurity, and that the peeling layer is made from a silicon oxide film doped with at least one impurity of boron, phosphorus and fluorine.
Thus, in removing the peeling layer through cleaning, selectivity against the insulating film can be attained, so that the peeling layer can be definitely removed.
In the second method for fabricating a semiconductor device, it is preferred that the insulating film is made from a silicon oxide film, and that the peeling layer is made from a silicon nitride film.
Thus, in removing the peeling layer through cleaning, selectivity against the insulating film can be attained, so that the peeling layer can be definitely removed.
In the second method for fabricating a semiconductor device, it is preferred that the underlying layer is made from a silicon nitride film, that the insulating film is made from a silicon oxide film, and that the peeling layer is made from a silicon film.
Thus, in removing the peeling layer through cleaning, selectivity against the insulating film can be attained, so that the peeling layer can be definitely removed. Also, the underlying layer can be prevented from being etched in removing the peeling layer.
The third method for fabricating a semiconductor device of this invention comprises the steps of depositing an insulating film on an underlying layer formed on a semiconductor substrate and depositing, on the insulating film, an implant stopper layer made from an insulating material harder than a material of the insulating film; forming a resist pattern on the implant stopper layer and subjecting the implant stopper layer and the insulating film to plasma etching using an etching gas including carbon and fluorine with the resist pattern used as a mask; performing ashing on a polymer film having been deposited on the resist pattern during the plasma etching by using an oxygen gas or a gas including oxygen as a principal constituent; and removing a fluorine implant layer having been formed in a surface portion of the implant stopper layer during the ashing.
In the third method for fabricating a semiconductor device, since the implant stopper layer is deposited on the insulating film, fluorine included in the polymer film is implanted into the implant stopper layer but not implanted into the insulating film. Furthermore, since the implant stopper layer is made from an insulating material harder than a material of the insulating film, fluorine generated from the polymer film is implanted into merely a shallow region but not implanted into a deep region in a surface portion of the implant stopper layer. Therefore, after removing the fluorine implant layer, surface roughness caused on the implant stopper layer is small. As a result, surface roughness derived from fluorine is never caused in the insulating film in wet cleaning subsequently performed.
Moreover, since the fluorine implant layer is removed, in the case where a resist film of a chemically amplified resist material is formed on the insulating film and the resist film is subjected to pattern exposure, an acid generated in an exposed portion of the resist film can be avoided from being deactivated through a reaction between an acid (H+) included in the chemically amplified resist material and fluorine.